1. Field of the Invention
This invention relates generally to the circuit design of a (.SIGMA.-.DELTA.) modulator and an automatic gain controller (AGC) implemented as integrated circuits(ICs) on a semiconductor chip. More particularly, this invention relates to an improved circuit design and architecture to more completely integrate a (.SIGMA.-.DELTA.) modulator with an automatic gain controller (AGC) circuit such that areas occupied by the circuits on a semiconductor chip are reduced and better signal to noise ratio are achieved when less signal distortions are generated by implementing the improved circuit design disclosed by this invention.
2. Description of the Prior Art
A circuit designer is faced with the difficulties that wastes of wafer areas on a semiconductor chip and a great deal of signal distortions are often resulted from applying a conventional circuit design architecture for implementing an automatic gain controller (AGC) and a sigma-delta modulator in an integrated circuit (IC) chip. Specifically, the automatic gain controller and the sigma-delta modulator are designed separately as two independent circuit elements. A multiplexer is then employed for interconnecting these two independent separate circuit elements for processing an output signals from the automatic gain controller (AGC) to generate an input signal to the sigma-delta modulator.
FIGS. 1 shows a functional block diagram of a conventional circuit implementation of an automatic gain controller (AGC) and a sigma-delta (.SIGMA.-.DELTA.) modulator. This conventional circuit implementation employs a dual-path architecture where the input signal is transmitted via a first path, i.e., path-A, to be first processed by a gain controller (AGC). The input signal is also transmitted via a second path, i.e., path-B, as a direct input signal. An output generated by the gain controller AGC and the direct input signal via the second path path-B are inputted to a multiplexer (MUX) and a signal is selected between two signals as an input to a sigma-delta (.SIGMA.-.DELTA.) modulator to complete the analog to digital conversion. As will be further explained below there are several difficulties and limitations imposed by this dual path configuration.
FIG. 2 shows a functional block of the automatic gain controller AGC of FIG. 1. The input signal to the AGC is first processed by a variable gain amplifier (VGA) and the output therefrom is then further processed by a gain amplifier A to generate an AGC output signal. The output signal is further inputted to a feedback loop which includes a high pass filter, a full wave rectifier and a peak detector to produce a feedback signal for inputting to the variable gain amplifier VGA to adjust the value of the gain such that the output of VGA is adjusted to have a fixed peak value. One definite disadvantage of this type of signal processing scheme is the reduction of the signal to noise ratio (SNR) when the amplitude of the input signal is increased. The reasons that the SNR is reduced with the increase of input signal will be further explained below when a comparison is made with the variations of the SNRs produced by a signal processing circuit of this invention.
Therefore, a need still exists in the art of circuit design and integrated circuit manufacture to provide a novel circuit design architecture to resolve the above difficulties. It is desirable that the circuit configuration can be conveniently implemented on a semiconductor chip as integrated circuited occupying less chip areas. It is further desirable that improved sigma-delta modulator can achieve better signal to noise ratios such that signal distortions produced can be significantly reduced.